1. Field of the Invention
The present invention relates to a clear ink, an ink jet recording method, an ink set, an ink cartridge, a recording unit and an ink jet recording apparatus.
2. Description of the Related Art
With the advancement of the ink jet recording technology in recent years, it has been possible to relatively easily and cheaply record such a high-definition image having high gloss brightness as realized in silver salt photography or offset printing even in ordinary homes.
When a dye ink containing a dye as a coloring material is used for realizing high gloss brightness, a good image free of graininess is obtained. On the other hand, however, the image involves a problem that fastness properties such as light fastness, water fastness and gas fastness are poor. Therefore, a pigment ink containing a pigment as a coloring material has come to be used in recent years. As such a pigment ink, is widely used a pigment ink with a pigment dispersed with a water-soluble polymer for the purpose of more improving fixability of the pigment to a recording medium and fastness properties of the resulting image, such as scratch resistance.
In recent years, it has been performed to record an image with plural pigment inks that have the same hue as each other and are different in the content of a coloring material for providing an image of high quality comparable with or superior to a silver salt photograph. However, when an image is recorded with such plural pigment inks, particularly, an ink relatively high in the content of the pigment (deep-color ink) and an ink relatively low in the content of the pigment (light-color ink), the resulting image involves problems that lowering of uniformity of gloss brightness and variability of gloss clarity result from differences in gloss brightness and smoothness by the kind of inks occur.
In order to solve such problems of lowering of the uniformity of gloss brightness and variability of the gloss clarity, various attempts have heretofore been made. For example, it has been proposed that when an ink is recorded with a deep-color ink and a light-color ink, the acid value of a polymer used in the deep-color ink is made lower than the acid value of a polymer used in the light-color ink (see Japanese Patent Application Laid-Open No. 2003-268275). It has also been proposed that a pigment ink and a clear ink containing polymer particles are applied to a recording medium, and the amounts of the pigment ink and clear ink ejected are controlled in such a manner that the gloss brightness on the whole surface of the recording medium becomes substantially uniform (see Japanese Patent Application Laid-Open No. 2003-335058).
It has further been proposed that polymer particles having a cross-linked core-shell structure are used as a polymer contained in a clear ink (see Japanese Patent Application Laid-Open No. 2008-179778). An ink set of a clear ink containing acrylic polymer fine particles and a pigment ink has also been proposed (see Japanese Patent Application Laid-Open No. 2007-276482).
It has further been proposed that polymer particles of a core-shell structure are caused to be contained in an ink containing a colorant, thereby improving smear resistance (Japanese Patent Application Laid-Open No. 2004-211089). In the polymer particles, acrylic esters of an alcohol having 1 to 50 carbon atoms and a substituted alcohol are used as hydrophobic monomers. Herein, it is described that the polymer particles are obtained by forming a core by general seed polymerization, then further applying a monomer to the core to form a shell and conducting cross-linking.